Exhaust throttle valve

ABSTRACT

An exhaust pressure control valve comprises a casing including a bore and a pair of first and second shaft support parts, and a valve shaft placed across the bore and rotatably supported by the shaft support parts. A first end portion of the valve shaft is arranged protruding outside the casing. A throttle valve is secured to the valve shaft inside the bore. An actuator is coupled to the first end portion to open and close the throttle valve. The second shaft support part is located in a bypass passage downstream from a bypass valve and has an opening in which a cap formed with a through hole is fitted. The inside of the second shaft support part is allowed to communicate with atmosphere through the through hole, the bypass passage, and others so that an end face of a second end portion can be subjected to atmospheric pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust throttle valve to be mountedin an exhaust system of an engine to restrict a flow of exhaust gas.

2. Description of Related Art

Heretofore, one technique of this type is disclosed in, for example, inJP2005-299457A. This '457A discloses an exhaust throttle valve mountedin an exhaust system of a diesel engine, downstream from a continuousregeneration DPF. As shown in FIG. 6, this exhaust throttle valve 61 isconfigured such that a butterfly valve element 64 is placed in a bore 63of a casing 62. The casing 62 is formed, at both sides (at right andleft sides in the figure), with a first shaft support part 66 and asecond shaft support part 67 corresponding to both end portions of avalve shaft 65 respectively. Both end portions 65 a and 65 b of thevalve shaft 65 are supported by the shaft support parts 66 and 67through bearings 68. One end (the first end portion) 65 a of the valveshaft 65 is provided protruding outside the casing 62 and connected toan actuator (not shown) through a lever 69. The other end (the secondend portion) 65 b of the valve shaft 65 is supported by the second shaftsupport part 67 through the bearing 68. An opening 67 a of the valvepart 67 at one end is closed by a cover plate 70.

However, in the exhaust throttle valve 61 disclosed in the above '457A,the opening 67 a of the second shaft support part 67 supporting thesecond end portion 65 b of the valve shaft 65 is closed by the coverplate 70. The inside of the shaft support part 67 is thus closed in bagor box form. Accordingly, when the valve element 64 is brought into afull closing position, the inner pressure of the bore 63 upstream fromthe valve element 64 rises, acting on the inside of the second shaftsupport part 67 through a gap between the valve shaft 65 and the bearing68. At that time, the first end portion 65 a of the valve shaft 65positioned outside the casing 62 is subjected to atmospheric pressure.This results in a difference in pressure acting on end faces of the endportions 65 a and 65 b of the valve shaft 65, thereby causing the valveshaft 65 to be pressed toward the first end portion 65 a. Consequently,the valve element 64 is also pressed toward the first end portion 65 a,and one side edge of the valve element 64 is pressed against an innerwall of the bore 63. When the valve element 64 in such a pressed stateis rotated, the inner wall of the bore 63 may be scratched due tofriction with the valve element 64. Such scratches are liable to causemalfunction of the valve element 64.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has an object to provide an exhaust throttle valve adapted tomitigate pressure that presses one side edge of a valve element againsta bore when the valve element is brought into a full closing position torestrict the flow of exhaust gas.

Additional objects and advantages of the invention will be set forth inpart in the description which follows and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve the purpose of the invention, there is provided an exhaustthrottle valve which can be mounted in an exhaust system of an engine torestrict a flow of exhaust gas, comprising: a casing including a boreand a pair of first shaft support part and second shaft support part; avalve shaft that has a first end portion and a second end portion and isplaced across the bore, the first end portion being rotatably supportedby the first shaft support part and the second end portion beingrotatably supported by the second shaft support part, the first endportion having one end passing through the first shaft support part toprotrude outside the casing; and a butterfly-type valve element attachedto the valve shaft inside the bore; wherein an inside of the secondshaft support part supporting the second end portion is allowed tocommunicate with atmosphere so that an end face of the second endportion can be subjected to atmospheric pressure.

According to another aspect, the invention provides an exhaust throttlevalve which can be mounted in an exhaust system of an engine to restricta flow of exhaust gas, comprising: a casing including a bore and a pairof first shaft support part and second shaft support part; a valve shaftthat has a first end portion and a second end portion and is placedacross the bore, the first end portion being rotatably supported by thefirst shaft support part and the second end portion being rotatablysupported by the second shaft support part, the first end portion havingone end passing through the first shaft support part to protrude outsidethe casing; and a butterfly-type valve element attached to the valveshaft inside the bore; and an actuator coupled with the end of the firstend portion protruding outside the casing and arranged to drive thevalve element to open and close, wherein an inside of the second shaftsupport part supporting the second end portion is allowed to communicatewith atmosphere so that an end face of the second end portion can besubjected to atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate an embodiment of the inventionand, together with the description, serve to explain the objects,advantages and principles of the invention.

In the drawings,

FIG. 1 is a schematic configuration view showing a configuration of anexhaust system of an engine;

FIG. 2 is a sectional view showing a schematic configuration of mainparts of an exhaust pressure control valve in a first embodiment;

FIG. 3 is a sectional view taken along a line A-A in FIG. 2;

FIG. 4 is a sectional view showing a schematic configuration of mainparts of an exhaust pressure control valve in a second embodiment;

FIG. 5 is a sectional view showing a schematic configuration of mainparts of an exhaust pressure control valve in a third embodiment; and

FIG. 6 is a sectional view sectional view showing a schematicconfiguration of main parts of an exhaust pressure control valve in aprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A detailed description of a first preferred embodiment of an exhaustthrottle valve embodying the present invention will now be givenreferring to the accompanying drawings. In this embodiment, the exhaustthrottle valve of the invention is embodied as an exhaust pressurecontrol valve for a diesel engine.

FIG. 1 is a schematic configuration view showing a configuration of anexhaust system of a diesel engine (hereinafter, simply referred to as an“engine”) 1. The exhaust system of the engine 1 includes a dieselparticulate filter (DPF) unit 2, an exhaust pressure control valve 3,and a muffler 4. Further, an electronic control unit (ECU) 6 is providedto control a fuel pump 5 of the engine 1 and the exhaust pressurecontrol valve 3. The DPF unit 2 contains a ceramic filter and oxidationcatalyst for trapping particulates and graphite (plumbago or black lead)contained in exhaust gas. An upstream side of the DPF unit 2 isconnected to an exhaust manifold 8 of the engine 1 through an exhaustpipe 7. A downstream side of the DPF unit 2 is connected to the exhaustpressure control valve 3 through an exhaust pipe 9. A downstream side ofthe exhaust pressure control valve 3 is connected to the muffler 4through an exhaust pipe 10. The upstream-side exhaust pipe 7 is attachedwith a first pressure sensor 11 to detect the pressure of exhaust gasand the downstream-side exhaust pip 9 is also attached with a secondpressure sensor 12 to detect the pressure of exhaust gas. The ECU 6 isarranged to receive a detection signal from each of the pressure sensors11 and 12 and detection signals from various sensors for detecting anoperating state of the engine 1. The ECU 6 controls the fuel pump 5 andthe exhaust pressure control valve 3 based on those detection signals.

The exhaust pressure control valve 3 is arranged to control the pressureof exhaust gas discharged out of the engine 1. In this embodiment, theexhaust pressure control valve 3 corresponds to an exhaust throttlevalve of the invention. The ECU 6 controls the fuel pump 5 according tothe operating state of the engine 1 to control a fuel supply amount anda fuel supply timing with respect to the engine 1. Further, the ECU 6calculates a difference in pressure (differential pressure) detected bythe pressure sensors 11 and 12. When the differential pressure exceeds apredetermined value, the ECU 6 closes the exhaust pressure control valve3 to regenerate a filter of the DPF unit 2.

Exhaust gas discharged out of the engine 1 flows to the DPF unit 2through the exhaust manifold 8 and the exhaust pipe 7. The DPF unit 2traps particulates and graphite contained in the exhaust gas to purifyor decontaminate the exhaust gas. The exhaust gas purified by the DPFunit 2 passes through the exhaust pipe 9, the exhaust pressure controlvalve 3, and the exhaust pipe 10 and is released to the atmospherethrough the muffler 4.

Herein, as the DPF unit 2 traps particulates and graphite, a pressureloss of the DPF unit 2 increases. This pressure loss appears as adifferential pressure between the upstream side and the downstream sideof the DPF unit 2. When ECU 6 determines that the differential pressureexceeds the predetermined value based on the detection signals from thepressure sensors 11 and 12, the ECU 6 closes the exhaust pressurecontrol valve 3. Accordingly, exhaust pressure of the engine 1 rises andan amount of fuel to be supplied to the engine 1 is increased accordingto the exhaust pressure rise. The exhaust gas containing unburnedcomponents therefore flows in the DPF unit 2, in which this gas issupplied to the oxidation catalyst placed upstream from the filter. Theunburned components supplied to the oxidation catalyst increase the gastemperature in the catalyst by oxidation reaction. This causes theparticulates and graphite trapped in the filter of the DPF unit 2 to beburnt, and thus the filter is regenerated. After completion ofregeneration of the filter of the DPF unit 2, the ECU 6 controls theexhaust pressure control valve 3 to open and returns to a normaloperation. Such regeneration control of the DPF unit 2 is executed everytime the pressure loss (differential pressure) of the DPF unit 2 exceedsthe predetermined value.

FIG. 2 is a sectional view showing a schematic configuration of mainparts of the exhaust pressure control valve 3. This exhaust pressurecontrol valve 3 is provided with a casing 23 including a bore 21 and abypass passage 22, a butterfly-type throttle valve element (hereinafter,“valve element”) 24 for opening and closing the bore 21, and a bypassvalve 25 for opening and closing the bypass passage 22. The valveelement 24 corresponds to a valve element of the invention. The bypassvalve 25 corresponds to a bypass valve element of the invention. Thebypass passage 22 is formed adjacent to in parallel with the bore 21 anddefined by a partition wall 26. An upstream side (a left side in thefigure) of the bore 21 is connected to the exhaust pipe 9 and adownstream side (a right side in the figure) of the bore 21 is connectedto the exhaust pipe 10. The partition wall 26 is formed with an inletport 22 a and an outlet port 22 b of the bypass passage 22. The inletport 22 a is formed to open in the bore 21 located upstream from thevalve element 24 and the outlet port 22 b is formed to open in the bore21 located downstream from the valve element 24. The valve element 24 isoperated to open or close the bore 21 between the inlet port 22 a andthe outlet port 22 b.

As shown in FIG. 2, the valve element 24 is a butterfly-type valveelement, which is secured to a valve shaft 27. In association withrotation of the valve shaft 27, the valve element 24 is selectivelyswitched to a position that fully opens the bore 21 or a position thatfully closes the bore 21. The bypass valve 25 is a flapper valve andsecured to a distal end of an arm 28 with a bolt 29. The arm 28 isrotatable about a pivot shaft 30. The arm 28 is rotated by an actuator(not shown) when the pressure of exhaust gas exceeds a predeterminedvalue, moving the bypass valve 25 to open the bypass passage 22.

FIG. 3 is a sectional view taken along a line A-A in FIG. 2. The inletport 22 a has a circular section so that the inlet port 22 a is easilygas-tightly closed by the bypass valve 25. The outlet port 22 b has anearly rectangular section to have a larger passage sectional area thanthe inlet port 22 a to facilitate the flow of exhaust gas through theoutlet port 22 b into the bore 21. As shown in FIG. 2, a downstream sideof the outlet port 22 b continuous with the bore 21 is formed as acurved portion 31, thereby allowing the exhaust gas flowing in the bore21 to easily flow toward the downstream side of the bore 21.

As shown in FIG. 2, the casing 3 further includes a pair of a firstshaft support part 32 and a second shaft support part 33. The valveshaft 27 is arranged to extend across the bore 21 and rotatablysupported by the first and second shaft support parts 32 and 33. Adistal end of an end portion (a first end portion) 27 a of the valveshaft 27 is arranged protruding outside the casing 23. The valve element24 is secured to the valve shaft 27 inside the bore 21. Specifically,part of the first end portion 27 a of the valve shaft 27 is rotatablysupported by the first shaft support part 32 through a bearing 34. Thisfirst shaft support part 33 is formed as a boss protruding outside thecasing 23. The distal end of the first end portion 27 a of the valveshaft 27 is placed passing through the first shaft support part 32 andprotruding outside. In the first shaft support part 32, a seal ring 35is set adjoining to the bearing 34. This seal ring 35 can preventleakage of exhaust gas. The distal end of the first end portion 27 aprotruding outside through the first shaft support part 32 is coupledwith a rod 37 of a diaphragm actuator 36 with a lever 38. The actuator36 is configured to operate when applied with negative pressure. Supplyof negative pressure to the actuator 36 is switched on/off by anunillustrated vacuum switching valve (VSV). The ECU 6 controls openingand closing of this VSV. When negative pressure is supplied to theactuator 36, causing the rod 37 to expand, the valve shaft 27 is rotatedby the lever 38. This rotation causes the valve element 24 to be closed.

As shown in FIG. 2, the other end portion (a second end portion) 27 b ofthe valve shaft 27 is rotatably supported by the second shaft supportpart 33 through a bearing 39. This second shaft support part 33 isformed as a boss in the partition wall 26 of the casing 23 and locatedbetween the inlet port 22 a and the outlet port 22 b. The second endportion 27 b of the valve shaft 27 is fitted and placed in the secondshaft support part 33. An opening 33 a of the second shaft support part33 opens in the bypass passage 22. In this shaft support part 33, a cap40 is fitted at the opening 33 a to prevent the particulates andgraphite from entering. This cap 40 also serves as a stopper forpreventing the bearing 39 from coming off the second shaft support part33. In this embodiment, the cap 40 is formed with a through hole 40 afor providing communication between the inside of the second shaftsupport part 33 and the bypass passage 22. This allows the inside of thesecond shaft support part 33 corresponding to the second end portion 27b of the valve shaft 27 to communicate with atmosphere through thebypass passage 22 and the exhaust pipe 10. Thus, atmosphere will acts onan end face 27 c of the second end portion 27 b through the bypasspassage 22. Accordingly, the pressure of exhaust gas to be applied onthe end face 27 c is greatly reduced. In measurements, while the valveelement 24 is closed, the pressure applied on the end face 27 c was “200kPa” in the case of the cap 40 formed with no through hole 40 a and thepressure applied on the end face 27 c was reduced to “60 kPa” in thecase of the cap 40 formed with the through hole 40 a.

According to the exhaust pressure control valve 3 in this embodimentmentioned above, when the control of regeneration of the DPF unit 2 isexecuted during operation of the engine 1, the valve element 24 of theexhaust pressure control valve 3 is switched from a fully open state toa fully closed state. Then, the inside of the bore 21 upstream from thevalve element 24 increases in pressure by the exhaust gas. This pressurewill act on the inside of the second shaft support part 33 supportingthe second end portion 27 b of the valve shaft 27 through a gap betweenthe bearing 39 and the valve shaft 27. In this embodiment, however, theinside of the second shaft support part 33 is allowed to communicatewith the atmosphere through the through hole 40 a of the cap 40, thebypass passage 22, and others. Accordingly, the inside of the secondshaft support part 33 will not remain at high pressure due to thepressure of exhaust gas and the atmospheric pressure will act on the endface 27 c of the second end portion 27 b. At that time, the first endportion 27 a of the valve shaft 27 is located outside the casing 23 andhence subjected to the atmospheric pressure. Both the end portions 27 aand 27 b of the valve shaft 27 are equally subjected to the atmosphericpressure. It is therefore possible to prevent the valve shaft 27 frombecoming pressed to one side in its axial direction and to greatlyreduce the pressure that presses one side edge of the valve element 24against the wall surface of the bore 21. As a result, the wall surfaceof the bore 21 can be protected from becoming scratched by friction withthe valve element 24. This makes it possible to prevent malfunction ofthe valve element 24 resulting from the scratches.

In this embodiment, the through hole 40 a is simply formed in the cap 40in order to make the inside of the second shaft support part 33communicate with the bypass passage 22. The need to machine the shaftsupport part 33 can be eliminated. Further, the through hole 40 a willnot impair the ability of the cap 40 to prevent the bearing 39 fromcoming off the second shaft support part 33.

In this embodiment, the bypass passage 22 is formed in the casing 23 bydetouring around the valve element 24. Accordingly, when the innerpressure of the bore 21 upstream from the valve element 24 rises due tothe exhaust gas, the bypass valve 25 is opened as appropriate to allowthe exhaust gas to flow in the bypass passage 22, so that the rise ininner pressure of the bore 21 upstream from the valve element 24 ismitigated. Consequently, the inner pressure of the bore 21 upstream fromthe valve element 24 can be controlled appropriately. The regenerationcontrol of the DPF unit 2 can therefore be executed appropriatelycontinuously since the start thereof.

In this embodiment, the inside of the second shaft support part 33supporting the second end portion 27 b of the valve shaft 27 is madecommunicate with the bypass passage 22 downstream from the bypass valve24. Thus, the inside of the second shaft support part 33 will notcommunicate directly with the outside. This makes it possible to preventdirect outside leakage of exhaust gas from the second shaft support part33 and maintain reliability as the exhaust pressure control valve 3.

Embodiment 2

A second embodiment of the exhaust throttle valve of the invention willbe explained below referring to the accompanying drawings. In eachembodiment explained below, similar parts or components to those in thefirst embodiment are given the same reference codes and their detailsare not explained repeatedly. The following explanation will be madewith a focus on differences from the first embodiment.

FIG. 4 is a sectional view of a schematic configuration of main parts ofthe exhaust pressure control valve 13 in this embodiment. In this secondembodiment, a bypass passage 41 is formed in the casing 23. However,this embodiment differs from the first embodiment in that the opening 33a of the second shaft support part 33 is not matched with the bypasspassage 41 and the opening 33 a of the second shaft support part 33 isnot positioned inside the bypass passage 41 but is placed outside thecasing 23. In FIG. 4, the bypass passage 41 includes an inlet port 41 aand an outlet port 41 b, and a bypass valve 25 is placed at the inletport 41 a. A diaphragm actuator 42 is placed close to the bypass passage41 to cause the bypass valve 25 to open and close.

Except that the exhaust gas may leak out through the through hole 40 aof the cap 40, this embodiment can provide the same operations andadvantages as those in the first embodiment.

Embodiment 3

A third embodiment of the exhaust throttle valve of the invention willbe explained in detail with reference to the accompanying drawing.

FIG. 5 is a sectional view of a schematic configuration of main parts ofan exhaust pressure control valve 14 in the third embodiment. Thisembodiment differs from the first and second embodiments in that thecasing 23 is not provided with any bypass passages 22 and 41, and theopening 33 a of the second shaft support part 33 is placed outside thecasing 23.

In this embodiment, consequently, the inner pressure of the bore 21upstream from the valve element 24 could not be controlledappropriately. Except for it, however, the third embodiment can providethe same operations and advantages as those in the first embodiment.

The present invention is not limited to the above embodiment(s) and maybe embodied in other specific forms without departing from the essentialcharacteristics thereof.

For instance, each of the above embodiments embody the exhaust throttlevalve of the invention as the exhaust pressure control valve 3, 13, or14 to be used for the control of regeneration of the DPF unit 2. As analternative, the exhaust throttle valve of the invention may be appliedto an exhaust throttle valve for exhaust brake.

While the presently preferred embodiment of the present invention hasbeen shown and described, it is to be understood that this disclosure isfor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

1. An exhaust throttle valve which can be mounted in an exhaust systemof an engine to restrict a flow of exhaust gas, comprising: a casingincluding a bore and a pair of first shaft support part and second shaftsupport part; a valve shaft that has a first end portion and a secondend portion and is placed across the bore, the first end portion beingrotatably supported by the first shaft support part and the second endportion being rotatably supported by the second shaft support part, thefirst end portion having one end passing through the first shaft supportpart to protrude outside the casing; and a butterfly-type valve elementattached to the valve shaft inside the bore; wherein an inside of thesecond shaft support part supporting the second end portion is allowedto communicate with atmosphere so that an end face of the second endportion can be subjected to atmospheric pressure.
 2. An exhaust throttlevalve which can be mounted in an exhaust system of an engine to restricta flow of exhaust gas, comprising: a casing including a bore and a pairof first shaft support part and second shaft support part; a valve shaftthat has a first end portion and a second end portion and is placedacross the bore, the first end portion being rotatably supported by thefirst shaft support part and the second end portion being rotatablysupported by the second shaft support part, the first end portion havingone end passing through the first shaft support part to protrude outsidethe casing; and a butterfly-type valve element attached to the valveshaft inside the bore; and an actuator coupled with the end of the firstend portion protruding outside the casing and arranged to drive thevalve element to open and close, wherein an inside of the second shaftsupport part supporting the second end portion is allowed to communicatewith atmosphere so that an end face of the second end portion can besubjected to atmospheric pressure.
 3. The exhaust throttle valveaccording to claim 1 further comprising: a bypass passage formed in thecasing to detour the valve element to provide communication between thebore upstream from the valve element and the bore downstream from thevalve; and a bypass valve element for opening and closing the bypasspassage.
 4. The exhaust throttle valve according to claim 2 furthercomprising: a bypass passage formed in the casing to detour the valveelement to provide communication between the bore upstream from thevalve element and the bore downstream from the valve; and a bypass valveelement for opening and closing the bypass passage.
 5. The exhaustthrottle valve according to claim 3, wherein the inside of the secondshaft support part supporting the second end portion is placed tocommunicate with the bypass passage downstream from the bypass valveelement.
 6. The exhaust throttle valve according to claim 4, wherein theinside of the second shaft support part supporting the second endportion is placed to communicate with the bypass passage downstream fromthe bypass valve element.
 7. The exhaust throttle valve according toclaim 1, wherein the second end portion is fitted in the second shaftsupport part and placed inside thereof, the first and second endportions are rotatably supported by the first and second shaft supportparts through bearings respectively.
 8. The exhaust throttle valveaccording to claim 2, wherein the second end portion is fitted in thesecond shaft support part and placed inside thereof, the first andsecond end portions are rotatably supported by the first and secondshaft support parts through bearings respectively.
 9. The exhaustthrottle valve according to claim 5, wherein the second end portion isfitted in the second shaft support part and placed inside thereof, thefirst and second end portions are rotatably supported by the first andsecond shaft support parts through bearings respectively.
 10. Theexhaust throttle valve according to claim 6, wherein the second endportion is fitted in the second shaft support part and placed insidethereof, the first and second end portions are rotatably supported bythe first and second shaft support parts through bearings respectively.11. The exhaust throttle valve according to claim 7, wherein at leastone of the first and second shaft support parts includes a seal ringadjoining to the bearing.
 12. The exhaust throttle valve according toclaim 8, wherein at least one of the first and second shaft supportparts includes a seal ring adjoining to the bearing.
 13. The exhaustthrottle valve according to claim 9, wherein at least one of the firstand second shaft support parts includes a seal ring adjoining to thebearing.
 14. The exhaust throttle valve according to claim 10, whereinat least one of the first and second shaft support parts includes a sealring adjoining to the bearing.
 15. The exhaust throttle valve accordingto claim 7, wherein the second shaft support part has an opening whichcan be subjected to the atmospheric pressure, and the exhaust throttlevalve further includes a cap fitted in the opening of the second shaftsupport part, the cap being formed with a through hole for providingcommunication between the inside and an outside of the second shaftsupport part so that the atmospheric pressure can act on an end face ofthe second end portion through the through hole.
 16. The exhaustthrottle valve according to claim 9, wherein the second shaft supportpart has an opening which can be subjected to the atmospheric pressure,and the exhaust throttle valve further includes a cap fitted in theopening of the second shaft support part, the cap being formed with athrough hole for providing communication between the inside and anoutside of the second shaft support part so that the atmosphericpressure can act on an end face of the second end portion through thethrough hole.
 17. The exhaust throttle valve according to claim 10,wherein the second shaft support part has an opening which can besubjected to the atmospheric pressure, and the exhaust throttle valvefurther includes a cap fitted in the opening of the second shaft supportpart, the cap being formed with a through hole for providingcommunication between the inside and an outside of the second shaftsupport part so that the atmospheric pressure can act on an end face ofthe second end portion through the through hole.
 18. The exhaustthrottle valve according to claim 11, wherein the second shaft supportpart has an opening which can be subjected to the atmospheric pressure,and the exhaust throttle valve further includes a cap fitted in theopening of the second shaft support part, the cap being formed with athrough hole for providing communication between the inside and anoutside of the second shaft support part so that the atmosphericpressure can act on an end face of the second end portion through thethrough hole.
 19. The exhaust throttle valve according to claim 13,wherein the second shaft support part has an opening which can besubjected to the atmospheric pressure, and the exhaust throttle valvefurther includes a cap fitted in the opening of the second shaft supportpart, the cap being formed with a through hole for providingcommunication between the inside and an outside of the second shaftsupport part so that the atmospheric pressure can act on an end face ofthe second end portion through the through hole.
 20. The exhaustthrottle valve according to claim 14, wherein the second shaft supportpart has an opening which can be subjected to the atmospheric pressure,and the exhaust throttle valve further includes a cap fitted in theopening of the second shaft support part, the cap being formed with athrough hole for providing communication between the inside and anoutside of the second shaft support part so that the atmosphericpressure can act on an end face of the second end portion through thethrough hole.